Why might I have an NT scan?

A screening test can only estimate the likelihood of your baby having Down syndrome, whereas a diagnostic test, such as CVS or amniocentesis, will tell you for sure. But diagnostic tests do carry a small risk of miscarriage.

An NT scan can't tell you for certain whether or not your baby is affected. What it can do is help you decide whether or not to have a diagnostic test.

How is the NT scan performed?

This ultrasound scan must take place at between 11 weeks and 13 weeks plus six days of pregnancy, or when your baby is between 45mm and 84mm.

Before this the scan is difficult to do because your baby is still so tiny. It would also be too early to combine it with your first trimester blood test.

After 14 weeks, any excess fluid may be absorbed by your baby's developing lymphatic system.

The scan is usually done through your tummy. But a vaginal scan may be needed, as it gives better views, especially if your uterus (womb) tilts backwards, or if you're overweight. This is quite safe for you and your baby, and it shouldn't be too uncomfortable either.

To date your pregnancy accurately, the sonographer will measure your baby from the top of his head to the bottom of his spine. She will then measure the width of the NT. The skin will appear as a white line, and the fluid under the skin will look black.

You'll be able to see your baby's head, spine, limbs, hands and feet on the screen. Some major abnormalities may be ruled out at this scan, such as abdominal wall and skull defects. But it's recommended that you also have a detailed anomaly scan at about 20 weeks.

It may help if your partner or a friend accompanies you, to share the experience and be with you when you get the result.

What is a normal measurement?

An NT of less than 3.5mm is considered normal when your baby measures between 45mm and 84mm (FASP 2010). The NT normally grows in proportion with your baby (Nicolaides et al 1999).

The image on the right shows a baby with a normal NT of 1.3mm. This baby has a low chance of having Down syndrome. But an increased NT doesn't mean there's definitely a problem. Some babies without Down syndrome have increased fluid, too.

The image on the left shows a baby with an NT of 2.9mm, which is within the limit of normal range. Nine out of 10 babies with a measurement between 2.5mm and 3.5mm won't have Down syndrome (Nicolaides et al 1999).

As the NT increases, so does the chance of Down syndrome and other chromosomal abnormalities.

The baby in the image to the left has an NT of 6mm and has a high chance of having Down syndrome, as well as other chromosomal abnormalities (Witters and Frynns 2007) and heart problems (Wald et al 2008).

Not many babies have as much fluid as this.

How is the likelihood of Down syndrome calculated?

Every woman has a chance of giving birth to a baby with Down syndrome. The chances increase with your age.

To calculate your likelihood, the sonographer will enter your age and the measurements from your NT scan into a database. It shows the likelihood that applies to all women of your age (the background risk).

Your scan measurements will be combined with your age to generate your own individual likelihood for this pregnancy. This may be higher or lower than your background likelihood. Many hospitals and private clinics offer a combined test, which combines the blood test results with the NT measurement.

What should I do if I get a high likelihood?

Most women with a high likelihood (one in 150 or less) will go on to have a baby without Down syndrome. Even with a likelihood as high as one in five, there's a four out of five chance that your baby doesn't have Down syndrome. Nevertheless, once your pregnancy has been labelled high-likelihood, you'll probably feel anxious and unsure of what to do.

The only way to know for certain if your baby has Down syndrome or another chromosomal abnormality is to have a diagnostic test, such as CVS or amniocentesis. This decision can be very difficult, but you don't have to decide in a hurry.

One advantage of the NT scan is that it's done early in your pregnancy. It's possible to have a CVS and get the result while you're still in your first trimester. If you're unsure, you can wait until 15 weeks and have an amniocentesis if you wish. Your midwife or doctor will provide more information if you need it.

Will I be given a likelihood level for any other abnormalities?

You'll also be given a likelihood level for Edwards syndrome and Patau syndrome, which are two other chromosomal abnormalities (Kagan et al 2008). These conditions are much rarer than Down syndrome, and most affected pregnancies do miscarry. Babies with these conditions usually also have abnormalities that can be seen on the scan.

How accurate is the NT scan?

The NT scan picks up about 75 per cent of babies with Down syndrome (Snijders et al 1998).

Sometimes, a screening test can suggest that a baby has a high likelihood of having Down syndrome, even though they don't actually have it. This is called a false positive. The false positive rate for an NT scan is five per cent. This means that one in 20 women is wrongly advised that they have a high likelihood.

Combining an NT scan with a blood test gives a more accurate result. The blood test measures the levels of the hormone free beta-hCG and the protein PAPP-A. Babies with Down syndrome tend to have high levels of hCG and low levels of PAPP-A.

When the NT scan is combined with this blood test, the detection rate increases to 90 per cent (Kagan et al 2008, Nicolaides et al 2000). This is called the combined test.

Where can I have an NT scan?

Your GP or obstetrician will refer you to an ultrasound clinic for the scan. Your blood tests may be completed by your caregiver or at a pathology clinic. Depending on the level of care you've chosen, you may have to pay for the scan and the blood tests.

Ask your GP or caregiver for more information about your options in your area.

Comments

My baby never slept well (especially through the night) until I started using the website >>SleepBaby.org<< - that website has been by far one of the best things I've ever got my hands on to get him to fall asleep quickly. Best time is 45 seconds from awake to asleep! Can’t imagine life without it! I heard about it through a kindergarten teacher who uses it to put to sleep a group of 30 children. Check it out! >>SleepBaby.org<< - sorry, you can't post links here so you'll have to turn it into a normal link :) Best of luck to you and your family!

I'm 23 and so is my fiance. This is my first baby and we have no family history of any kind of disability or abnormalities. Today i was told i'm at a higher risk of one of the 3 abnormalities that they test for (cant remember which one) it was about 1 in 301 chance. Somebody please help me put my mind at ease!

I'm 33yrs. I have my NT scan at 12 weeks, my results is 2.3mm. The GP said is a little bit high. I was totally freaked out. I fell a lot more better after read through this article and found out anything under 3.5mm is considered normal. Still waiting for the combination result. God bless.

If it is only a NT scan you will be fine. The amniocentesis carries a very very small risk of miscarriage. Being 15 weeks I dont think they will do an NT scan now as its a bit late so if it's not going to affect the outcome of your decision, dont worry about it. If you would terminate based on the results than go with further testing like the amniocentesis.

I am 15 weeks pregnant with my second child. Have been reccomend to go get a nuchal transluceny scan. I am a little worried as this a can cause a miscarriage. Would like to hear from people who have had one of these test done. I am 36 years old and had a etopic pregnancy 14 months ago. So this wiĺl be my last pregnancy. We already have a special needs son.

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